This invention relates to a process for preparing high internal phase ratio emulsions and artificial latexes derived therefrom.
In conventional emulsions, the dispersed phase contains spheres having a volume fraction of less than 0.74, which is the volume fraction of the most compact arrangement of spheres of equal radius. However, for high internal phase ratio (HIPR) emulsions, the dispersed phase may have a dispersed phase volume fraction as high as 0.99. The continuous phase in such cases forms a thin liquid film that separate polyhedral cells, and the HIPR emulsion is stabilized by the adsorption of surfactant from the continuous phase on the surface of the cells. These HIPR emulsions are known to be useful as precursors to high molecular weight polymers, composites, and membranes for separation systems. (See Ruckenstein et al., J. Colloid Interface Science, Vol. 133, No. 2, p. 432 (1989), incorporated herein by reference.)
The preparation of HIPR emulsions by a continuous process is known. For example, in U.S. Pat. No. 4,018,426 (incorporated herein by reference), Mertz et al. discloses a system for the continuous production of HIPR emulsions wherein two precisely metered immiscible phases (an internal phase and an inverter phase formed of an internal-in-external emulsifier and an external phase) are mixed together in a preselected ratio of internal phase to inverter phase of between 95 to 5 and 75 to 25 weight percent. The system provides a mixing means receiving the separate streams of the internal phase and the inverter phase and distributing loosely the internal phase in stepwise progression into the inverter phase. This stepwise progression is formed by positioning a plate containing a plurality of holes of progressively larger size which serve to distribute the internal phase stepwise in progressively greater total amount into the external phase before the resultant mixture reaches an outlet. Emulsions provided by the system disclosed by Mertz et al. are described as being useful in the pharmaceutical, food, cosmetic, and firefighting industries. It would be an advance in the art to provide a simple means of preparing HIPR emulsions.
The above-cited references do not suggest that HIPR emulsions can be used to prepare artificial latexes. Artificial latexes are conventionally prepared by the steps of: 1) providing a solution of a polymer or copolymer in a volatile organic solvent; 2) emulsifying the solution with water and an aqueous emulsifier; 3) removing the volatile organic solvent from the emulsion; and 4) adjusting the solids content if desired. Preparations of artificial latexes are described in U.S. Pat. Nos. 3,360,599; 3,503,917; 4,123,403; and 5,037,864 all incorporated herein by reference.
In the emulsification and water addition step of the preparation of the artificial latex by a phase inversion process, water and emulsifier are typically slowly added to a polymer solution with high shear until a prescribed water to polymer solution ratio is reached, generally in the range from 1:10 to 4:1. Continuous processes are also known. For example, European Patent Application No. 0283247 A2 discloses a continuous method for dispersing a hydrophobic substance in water at elevated temperature and pressure to prepare an artificial latex having a particle size of less than 0.5 micron.
There remains a need to prepare monodisperse latexes having submicron particle size, especially in the field of coating applications, where controlled particle size, minimization of creaming, and improved latex stability are desirable.